How long can semen leak from a vagina after having sex?

I made a profile on this site to get an answer to one specific question that i really need the answer to. I know that semen can leak from a vagina after sex it happens to my girlfriend all the time. what i'm wondering is how long can this happen? can it continue to leak for days and days? the reason i'm asking is because we have not had sex for about six days and i found semen stains on the underwear she's been wearing every day this week. i'm beginning to think shes cheating but i need to know for sure. somebody PLEASE give me an answer!